The present invention relates to cascaded instrument-type potential transformers for developing a low secondary voltage which is proportional to the high primary voltage and particularly to a method for compensating the transformer voltage ratio so that the magnitude of the secondary voltage reflects the primary voltage within acceptable accuracy tolerance limits over the entire operating voltage range.
Instrument potential transformers are widely used by electrical utilities to develop a low secondary voltage for application to indicating and/or recording equipment so as to provide a measure of the high primary or line voltage. These transformers are also utilized to power lower voltage relays and other control devices, while effectively isolating these devices and instruments from the high primary voltage. In the case of voltage measuring equipment, it is critical, in terms of meeting customer demands and satisfying established industry standards, that the voltage transformation from a high primary voltage to a low secondary voltage have a voltage ratio error tolerance of no more than plus or minum 0.003. That is, errors in the secondary voltage magnitude are manifested in a ratio of primary to secondary voltage which is slightly less or slightly greater than the desired or nameplate ratio as determined by the ratio of primary to secondary winding turns. Out of tolerance voltage ratio error can arise even though stringent quality control checks are made to insure that the correct number of primary turns, which number in the tens and hundreds of thousands for kilovolt ratings of 100 and above, are indeed wound. Typically, the core itself is the major source of ratio error. The problem is aggravated at the high kilovolt ratings in that instrument transformers are typically built with two or even three distinct core stages arranged in well spaced, vertically cascaded fashion. Even slight variations in the core magnetic reluctance and/or losses of the core in each of the various stages will produce inequalities in the voltage drops across the several primary windings and consequent voltage ratio errors. A contributing factor can be unequal stray capacitances between each stage and ground. Since the output or secondary winding is wound about the core of only one stage, usually the one adjacent the low voltage or grounded terminal of the transformer, an inequality in the voltage drop across the primary winding of that stage relative to the voltage drops across the primary winding of the other stage or stages will result in an induced secondary winding voltage which is not precisely equal to its turns ratio relationship to the primary winding circuit.
An unacceptable voltage ratio error can not be identified until the voltage transformer is completely built and fully tested at full and ten percent over full excitation. Preliminary tests at lower excitation voltages typically do not reveal a problem transformer. The conventional remedy to correct voltage ratio errors is disassemble the core or cores and the coils and restack the core laminations and/or rework the cores. Since cascaded potential transformers are encased in a porcelain housing filled with insulating oil and can be up to sixteen feet tall, this voltage ratio correction procedure is a time consuming, expensive operation.
It is accordingly an object of the present invention to improve the voltage ratio accuracy of instrument potential transformers.
A further object is to provide an improved method of correcting for voltage ratio errors in multi-stage, cascaded instrument potential transformers.
An additional object of the present invention is to provide a voltage ratio correction method of the above character which obviates the need for restacking and or reworking the cores of a cascaded instrument potential transformer.
Yet another object of the present invention is to provide a voltage ratio correction method of the above-character which is effective and is efficient and practical in its execution.
Other objects of the invention will in part be obvious and in part appear hereinafter.